adsorption and-aggregation-of-surf

• 1. ADSORPTION ANDAGGREGATION OFSURFACTANTS INSOLUTIONedited byK. L. MittalHopewell Junction, New York, U.S.A.Dinesh O. ShahUniversity of FloridaGainesville, Florida, U.S.A.Marcel Dekker, Inc. New York BaselCopyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

2. ISBN: 0-8247-0843-1This book is printed on acid-free paper.HeadquartersMarcel Dekker, Inc.270 Madison Avenue, New York, NY 10016tel: 212-696-9000; fax: 212-685-4540Eastern Hemisphere DistributionMarcel Dekker AGHutgasse 4, Postfach 812, CH-4001 Basel, Switzerlandtel: 41-61-260-6300; fax: 41-61-260-6333World Wide Webhttp://www.dekker.comThe publisher offers discounts on this book when ordered in bulk quantities. Formore information, write to Special Sales/Professional Marketing at the headquartersaddress above.Copyright2003 by Marcel Dekker, Inc. All Rights Reserved.Neither this book nor any part may be reproduced or transmitted in any form or byany means, electronic or mechanical, including photocopying, microfilming, andrecording, or by any information storage and retrieval system, without permissionin writing from the publisher.Current printing (last digit):10 9 8 7 6 5 4 3 2 1PRINTED IN THE UNITED STATES OF AMERICA 3. SURFACTANT SCIENCE SERIESFOUNDING EDITORMARTIN J. SCHICK19181998SERIES EDITORARTHUR T. HUBBARDSanta Barbara Science ProjectSanta Barbara, CaliforniaADVISORY BOARDDANIEL BLANKSCHTEINDepartment of Chemical EngineeringMassachusetts Institute of TechnologyCambridge, MassachusettsS. KARABORNIShell International PetroleumCompany LimitedLondon, EnglandLISA B. QUENCERThe Dow Chemical CompanyMidland, MichiganJOHN F. SCAMEHORNInstitute for Applied Surfactant ResearchUniversity of OklahomaNorman, OklahomaP. SOMASUNDARANHenry Krumb School of MinesColumbia UniversityNew York, New YorkERICW. KALERDepartment of Chemical EngineeringUniversity of DelawareNewark, DelawareCLARENCE MILLERDepartment of Chemical EngineeringRice UniversityHouston, TexasDON RUBINGHThe ProcterGamble CompanyCincinnati, OhioBEREND SMITShell International Oil Products B.V.Amsterdam, The NetherlandsJOHN TEXTERStrider Research CorporationRochester, New York 4. 1. Nonionic Surfactants, edited by Martin J. Schick (see also Volumes 19, 23, and 60)2. Solvent Properties of Surfactant Solutions, edited by Kozo Shinoda (see Volume 55)3. Surfactant Biodegradation, R. D. Swisher (see Volume 18)4. Cationic Surfactants, edited by Eric Jungermann (see also Volumes 34, 37, and 53)5. Detergency: Theory and Test Methods (in three parts), edited by W. G. Cutler and R.C. Davis (see also Volume 20)6. Emulsions and Emulsion Technology (in three parts), edited by Kenneth J. Lissant7. Anionic Surfactants (in two parts), edited by Warner M. Linfield (see Volume 56)8. Anionic Surfactants: Chemical Analysis, edited by John Cross9. Stabilization of Colloidal Dispersions by Polymer Adsorption, Tatsuo Sato andRichard Ruch10. Anionic Surfactants: Biochemistry, Toxicology, Dermatology, edited by ChristianGloxhuber (see Volume 43)11. Anionic Surfactants: Physical Chemistry of Surfactant Action, edited by E. H.Lucassen-Reynders12. Amphoteric Surfactants, edited by B. R. Bluestein and Clifford L. Hilton (see Volume59)13. Demulsification: Industrial Applications, Kenneth J. Lissant14. Surfactants in Textile Processing, Arved Datyner15. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications,edited by Ayao Kitahara and Akira Watanabe16. Surfactants in Cosmetics, edited by Martin M. Rieger (see Volume 68)17. Interfacial Phenomena: Equilibrium and Dynamic Effects, Clarence A. Miller and P.Neogi18. Surfactant Biodegradation: Second Edition, Revised and Expanded, R. D. Swisher19. Nonionic Surfactants: Chemical Analysis, edited by John Cross20. Detergency: Theory and Technology, edited by W. Gale Cutler and Erik Kissa21. Interfacial Phenomena in Apolar Media, edited by Hans-Friedrich Eicke and GeoffreyD. Parfitt22. Surfactant Solutions: New Methods of Investigation, edited by Raoul Zana23. Nonionic Surfactants: Physical Chemistry, edited by Martin J. Schick24. Microemulsion Systems, edited by Henri L. Rosano and Marc Clausse25. Biosurfactants and Biotechnology, edited by Naim Kosaric, W. L. Cairns, and Neil C.C. Gray26. Surfactants in Emerging Technologies, edited by Milton J. Rosen27. Reagents in Mineral Technology, edited by P. Somasundaran and Brij M. Moudgil28. Surfactants in Chemical/Process Engineering, edited by Darsh T. Wasan, Martin E.Ginn, and Dinesh O. Shah29. Thin Liquid Films, edited by I. B. Ivanov30. Microemulsions and Related Systems: Formulation, Solvency, and PhysicalProperties, edited by Maurice Bourrel and Robert S. Schechter31. Crystallization and Polymorphism of Fats and Fatty Acids, edited by Nissim Garti andKiyotaka Sato32. Interfacial Phenomena in Coal Technology, edited by Gregory D. Botsaris and Yuli M.Glazman33. Surfactant-Based Separation Processes, edited by John F. Scamehorn and Jeffrey H.Harwell34. Cationic Surfactants: Organic Chemistry, edited by James M. Richmond35. Alkylene Oxides and Their Polymers, F. E. Bailey, Jr., and Joseph V. Koleske36. Interfacial Phenomena in Petroleum Recovery, edited by Norman R. Morrow37. Cationic Surfactants: Physical Chemistry, edited by Donn N. Rubingh and Paul M.Holland38. Kinetics and Catalysis in Microheterogeneous Systems, edited by M. Grtzel and K.Kalyanasundaram39. Interfacial Phenomena in Biological Systems, edited by Max Bender40. Analysis of Surfactants, Thomas M. Schmitt (see Volume 96) 5. 41. Light Scattering by Liquid Surfaces and Complementary Techniques, edited byDominique Langevin42. Polymeric Surfactants, Irja Piirma43. Anionic Surfactants: Biochemistry, Toxicology, Dermatology. Second Edition, Revisedand Expanded, edited by Christian Gloxhuber and Klaus Knstler44. Organized Solutions: Surfactants in Science and Technology, edited by Stig E.Friberg and Bjrn Lindman45. Defoaming: Theory and Industrial Applications, edited by P. R. Garrett46. Mixed Surfactant Systems, edited by Keizo Ogino and Masahiko Abe47. Coagulation and Flocculation: Theory and Applications, edited by Bohuslav Dobi48. Biosurfactants: Production Properties Applications, edited by Naim Kosaric49. Wettability, edited by John C. Berg50. Fluorinated Surfactants: Synthesis Properties Applications, Erik Kissa51. Surface and Colloid Chemistry in Advanced Ceramics Processing, edited by RobertJ. Pugh and Lennart Bergstrm52. Technological Applications of Dispersions, edited by Robert B. McKay53. Cationic Surfactants: Analytical and Biological Evaluation, edited by John Cross andEdward J. Singer54. Surfactants in Agrochemicals, Tharwat F. Tadros55. Solubilization in Surfactant Aggregates, edited by Sherril D. Christian and John F.Scamehorn56. Anionic Surfactants: Organic Chemistry, edited by Helmut W. Stache57. Foams: Theory, Measurements, and Applications, edited by Robert K. Prud'hommeand Saad A. Khan58. The Preparation of Dispersions in Liquids, H. N. Stein59. Amphoteric Surfactants: Second Edition, edited by Eric G. Lomax60. Nonionic Surfactants: Polyoxyalkylene Block Copolymers, edited by Vaughn M. Nace61. Emulsions and Emulsion Stability, edited by Johan Sjblom62. Vesicles, edited by Morton Rosoff63. Applied Surface Thermodynamics, edited by A. W. Neumann and Jan K. Spelt64. Surfactants in Solution, edited by Arun K. Chattopadhyay and K. L. Mittal65. Detergents in the Environment, edited by Milan Johann Schwuger66. Industrial Applications of Microemulsions, edited by Conxita Solans and HironobuKunieda67. Liquid Detergents, edited by Kuo-Yann Lai68. Surfactants in Cosmetics: Second Edition, Revised and Expanded, edited by Martin M.Rieger and Linda D. Rhein69. Enzymes in Detergency, edited by Jan H. van Ee, Onno Misset, and Erik J. Baas70. Structure-Performance Relationships in Surfactants, edited by Kunio Esumi andMinoru Ueno71. Powdered Detergents, edited by Michael S. Showell72. Nonionic Surfactants: Organic Chemistry, edited by Nico M. van Os73. Anionic Surfactants: Analytical Chemistry, Second Edition, Revised and Expanded,edited by John Cross74. Novel Surfactants: Preparation, Applications, and Biodegradability, edited by KristerHolmberg75. Biopolymers at Interfaces, edited by Martin Malmsten76. Electrical Phenomena at Interfaces: Fundamentals, Measurements, and Applications,Second Edition, Revised and Expanded, edited by Hiroyuki Ohshima and KunioFurusawa77. Polymer-Surfactant Systems, edited by Jan C. T. Kwak78. Surfaces of Nanoparticles and Porous Materials, edited by James A. Schwarz andCristian I. Contescu79. Surface Chemistry and Electrochemistry of Membranes, edited by Torben SmithSrensen80. Interfacial Phenomena in Chromatography, edited by Emile Pefferkorn 6. 81. SolidLiquid Dispersions, Bohuslav Dobi, Xueping Qiu, and Wolfgang von Rybinski82. Handbook of Detergents, editor in chief: Uri ZollerPart A: Properties, edited by Guy Broze83. Modern Characterization Methods of Surfactant Systems, edited by Bernard P. Binks84. Dispersions: Characterization, Testing, and Measurement, Erik Kissa85. Interfacial Forces and Fields: Theory and Applications, edited by Jyh-Ping Hsu86. Silicone Surfactants, edited by Randal M. Hill87. Surface Characterization Methods: Principles, Techniques, and Applications, editedby Andrew J. Milling88. Interfacial Dynamics, edited by Nikola Kallay89. Computational Methods in Surface and Colloid Science, edited by MagorzataBorwko90. Adsorption on Silica Surfaces, edited by Eugne Papirer91. Nonionic Surfactants: Alkyl Polyglucosides, edited by Dieter Balzer and HaraldLders92. Fine Particles: Synthesis, Characterization, and Mechanisms of Growth, edited byTadao Sugimoto93. Thermal Behavior of Dispersed Systems, edited by Nissim Garti94. Surface Characteristics of Fibers and Textiles, edited by Christopher M. Pastore andPaul Kiekens95. Liquid Interfaces in Chemical, Biological, and Pharmaceutical Applications, edited byAlexander G. Volkov96. Analysis of Surfactants: Second Edition, Revised and Expanded, Thomas M. Schmitt97. Fluorinated Surfactants and Repellents: Second Edition, Revised and Expanded,Erik Kissa98. Detergency of Specialty Surfactants, edited by Floyd E. Friedli99. Physical Chemistry of Polyelectrolytes, edited by Tsetska Radeva100. Reactions and Synthesis in Surfactant Systems, edited by John Texter101. Protein-Based Surfactants: Synthesis, Physicochemical Properties, and Applications,edited by Ifendu A. Nnanna and Jiding Xia102. Chemical Properties of Material Surfaces, Marek Kosmulski103. Oxide Surfaces, edited by James A. Wingrave104. Polymers in Particulate Systems: Properties and Applications, edited by Vincent A.Hackley, P. Somasundaran, and Jennifer A. Lewis105. Colloid and Surface Properties of Clays and Related Minerals, Rossman F. Gieseand Carel J. van Oss106. Interfacial Electrokinetics and Electrophoresis, edited by ngel V. Delgado107. Adsorption: Theory, Modeling, and Analysis, edited by Jzsef Tth108. Interfacial Applications in Environmental Engineering, edited by Mark A. Keane109. Adsorption and Aggregation of Surfactants in Solution, edited by K. L. Mittal andDinesh O. Shah110. Biopolymers at Interfaces: Second Edition, Revised and Expanded, edited by MartinMalmsten111. Biomolecular Films: Design, Function, and Applications, edited by James F. Rusling112. StructurePerformance Relationships in Surfactants: Second Edition, Revised andExpanded, edited by Kunio Esumi and Minoru UenoADDITIONAL VOLUMES IN PREPARATIONLiquid Interfacial Systems: Oscillations and Instability, Rudolph V. Birikh, Vladimir A.Briskman, Manuel G. Velarde, and Jean-Claude Legros 7. Novel Surfactants: Preparation, Applications, and Biodegradability: Second Edition,Revised and Expanded, edited by Krister HolmbergColloidal Polymers: Preparation and Biomedical Applications, edited by AbdelhamidElaissari 8. iiiPrefaceThis volume embodies, in part, the proceedings of the 13th InternationalSymposium on Surfactants in Solution (SIS) held in Gainesville, Florida,June 1116, 2000. The theme of this particular SIS was Surfactant Scienceand Technology for the New Millennium. The final technical program com-prised360 papers, including 96 poster presentations, which was a testimonialto the brisk research activity in the arena of surfactants in solution. In lightof the legion of papers, to chronicle the total account of this event wouldhave been impractical, so we decided to document only the plenary andinvited presentations. The contributors were asked to cover their topics in ageneral manner; concomitantly, this book reflects many excellent reviews ofa number of important ramifications of surfactants in solution.Chapters 14 document the plenary lectures, including the written ac-countof the special Host Lecture by one of us (DOS) and Prof. BrijMoudgil. Chapters 532 embody the text of 28 invited presentations cov-eringmany aspects of surfactants in solution. Among the topics covered are:surfactant-stabilized particles; solid particles at liquid interfaces; nanocap-sules;aggregation behavior of surfactants; micellar catalysis; vesicles andliposomes; the clouding phenomenon; viscoelasticity of micellar solutions;phase behavior of microemulsions; reactions in microemulsions; viscosityindex improvers; foams, foam films, and monolayers; principles of emulsionformulation engineering; nano-emulsions; liposome gene delivery; poly-mericsurfactants; and combinatorial surface chemistry.As surfactants play an important role in many and diverse technologies,ranging from high-tech (microelectronics) to low-tech (washing clothes) ap-plications,an understanding of their behavior in solution is of paramount 9. iv Prefaceimportance. Also, as we learn more about surfactants and devise new sur-factantformulations, novel and exciting applications will emerge.The present compendium of excellent overviews and research papers pro-videsa bounty of up-to-date information on the many and varied aspects ofsurfactants in solution. It also offers a commentary on current research ac-tivityregarding the behavior of surfactants in solution. We hope that anyoneinvolved or interestedcentrally or tangentiallyin surfactants will findthis book useful. Further, we trust that both veteran researchers and thoseembarking on their maiden voyage in the wonderful world of surfactantswill find this treatise valuable.To put together a symposium of this magnitude and quality requires ded-icationand unflinching help from a battalion of people, and now it is ourpleasure and duty to acknowledge those who helped in many and variedmanners in this endeavor. First and foremost, we express our heartfelt andmost sincere thanks to Prof. Brij Moudgil, Director of the Engineering Re-searchCenter for Particle Science and Technology, University of Florida,for helping in more ways than one. He wore many different hatsas co-chairman,as troubleshooter, as local hostand he was always ready andwilling to help with a smile. Next we are thankful to faculty members,postdoctoral associates, graduate students, and administrative staff of boththe Center for Surface Science and Engineering and the Engineering Re-searchCenter for Particle Science and Technology, University of Florida.We acknowledge the generous support of the following organizations: theFlorida Institute of Phosphate Research, the National Science Foundation,and the University of Florida. Many individual industrial corporations helpedus by providing generous financial support and we are grateful to them. Wealso thank the exhibitors of scientific instruments and books for their con-tributionand support.We are grateful to the authors for their interest, enthusiasm, and contri-butionwithout which this book would not have seen the light of day. Last,we are appreciative of the efforts of the staff at Marcel Dekker, Inc. forgiving this book a body form.K. L. MittalDinesh O. Shah 10. vContentsPreface iiiContributors ix1. Highlights of Research on Molecular Interactions at Interfaces fromthe University of Florida 1Dinesh O. Shah and Brij M. Moudgil2. Interaction Between Surfactant-Stabilized Particles:Dynamic Aspects 49J. Lyklema3. Solid Particles at Liquid Interfaces, Including Their Effects onEmulsion and Foam Stability 61Robert Aveyard and John H. Clint4. From Polymeric Films to Nanocapsules 91Helmuth Mohwald, Heinz Lichtenfeld, Sergio Moya, A. Voight,G. B. Sukhorukov, Stefano Leporatti, L. Dahne, Igor Radtchenko,Alexei A. Antipov, Changyou Gao, and Edwin Donath5. Investigation of Amphiphilic Systems by Subzero TemperatureDifferential Scanning Calorimetry 105Shmaryahu Ezrahi, Abraham Aserin, and Nissim Garti6. Aggregation Behavior of Dimeric and Gemini Surfactants inSolution: Raman, Selective Decoupling 13C NMR, andSANS Studies 133Hirofumi Okabayashi, Norikatsu Hattori, and Charmian J. OConnor 11. vi Contents7. Snared by Trapping: Chemical Explorations of InterfacialCompositions of Cationic Micelles 149Laurence S. Romsted8. Effect of Surfactants on Pregastric EnzymeCatalyzed Hydrolysis ofTriacylglycerols and Esters 171Charmian J. OConnor, Douglas T. Lai, and Cynthia Q. Sun9. Effect of Benzyl Alcohol on the Properties of CTAB/KBrMicellar Systems 189Ganzuo Li, Weican Zhang, Li-Qiang Zheng, and Qiang Shen10. Vesicle Formation by Chemical Reactions: Spontaneous VesicleFormation in Mixtures of Zwitterionic and CatanionicSurfactants 201Klaus Horbaschek, Michael Gradzielski, and Heinz Hoffmann11. Mechanism of the Clouding Phenomenon in SurfactantSolutions 211C. Manohar12. Atomic Force Microscopy of Adsorbed Surfactant Micelles 219William A. Ducker13. A Simple Model to Predict Nonlinear Viscoelasticity and ShearBanding Flow of Wormlike Micellar Solutions 243J. E. Puig, F. Bautista, J. H. Perez-Lopez, J. F. A. Soltero, andOctavio Manero14. Preparation and Stabilization of Silver Colloids in AqueousSurfactant Solutions 255Dae-Wook Kim, Seung-Il Shin, and Seong-Geun Oh15. Silver and Palladium Nanoparticles Incorporated in LayerStructured Materials 269Rita Patakfalvi, Szilvia Papp, and Imre Dekany16. Water-in-Carbon Dioxide Microemulsions Stabilized byFluorosurfactants 299Julian Eastoe, Alison Paul, David Steytler, Emily Rumsey,Richard K. Heenan, and Jeffrey Penfold17. Organic Synthesis in Microemulsions: An Alternative or aComplement to Phase Transfer Catalysis 327Krister Holmberg and Maria Hager 12. Contents vii18. Physicochemical Characterization of Nanoparticles Synthesizedin Microemulsions 343J. B.Nagy, L. Jeunieau, F. Debuigne, and I. Ravet-Bodart19. Phase Behavior of Microemulsion Systems Based on OptimizedNonionic Surfactants 387Wolfgang von Rybinski and Matthias Wegener20. Microemulsions in Foods: Challenges and Applications 407Anilkumar G. Gaonkar and Rahul Prabhakar Bagwe21. Microemulsion-Based Viscosity Index Improvers 431Surekha Devi and Naveen Kumar Pokhriyal22. Foams, Foam Films, and Monolayers 453Dominique Langevin23. Role of Entry Barriers in Foam Destruction by Oil Drops 465Asen D. Hadjiiski, Nikolai D. Denkov, Slavka S. Tcholakova, andIvan B. Ivanov24. Principles of Emulsion Formulation Engineering 501Jean-Louis Salager, Laura Marquez, Isabel Mira, Alejandro Pena,Eric Tyrode, and Noelia B. Zambrano25. Nano-Emulsions: Formation, Properties, and Applications 525Conxita Solans, Jordi Esquena, Ana Maria Forgiarini, Nuria Uson,Daniel Morales, Paqui Izquierdo, Nuria Azemar, andMara Jose Garca-Celma26. Surface Modifications of Liposomes for Recognition and Responseto Environmental Stimuli 555Jong-Duk Kim, Soo Kyoung Bae, Jin-Chul Kim, and Eun-Ok Lee27. Specific Partition of Surface-Modified Liposomes in AqueousPEO/Polysaccharide Two-Phase Systems 579Eui-Chul Kang, Kazunari Akiyoshi, and Junzo Sunamoto28. Novel Cationic Transfection Lipids for Use in LiposomalGene Delivery 603Rajkumar Banerjee, Prasanta Kumar Das,Gollapudi Venkata Srilakshmi, Nalam Madhusudhana Rao, andArabinda Chaudhuri29. Combinatorial Surface Chemistry: A Novel Concept for Langmuirand LangmuirBlodgett Films Research 619Qun Huo and Roger M. Leblanc 13. viii Contents30. Oscillating Structural Forces Reflecting the Organization of BulkSolutions and Surface Complexes 635Per M. Claesson and Vance Bergeron31. Effect of Polymeric Surfactants on the Behavior of PolycrystallineMaterials with Special Reference to Ammonium Nitrate 655Arun Kumar Chattopadhyay32. Surface Tension Measurements with Top-Loading Balances 675Brian Grady, Andrew R. Slagle, Linda Zhu, Edward E. Tucker,Sherril D. Christian, and John F. ScamehornIndex 689 14. ixContributorsKazunari Akiyoshi Department of Synthetic Chemistry and BiologicalChemistry, Kyoto University, Kyoto, JapanAlexei A. Antipov Max-Planck-Institute of Colloids and Interfaces, Pots-dam,GermanyAbraham Aserin Casali Institute of Applied Chemistry, The HebrewUniversity of Jerusalem, Jerusalem, IsraelRobert Aveyard Department of Chemistry, Hull University, Hull, UnitedKingdomNu ria Azemar Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, SpainSoo Kyoung Bae Department of Chemical and Biomolecular Engineer-ing,KAIST, Daejeon, KoreaRahul Prabhakar Bagwe Department of Chemical Engineering, Uni-versityof Florida, Gainesville, Florida, U.S.A.Rajkumar Banerjee* Division of Lipid Science and Technology, IndianInstitute of Chemical Technology, Hyderabad, India*Current affiliation: University of Pittsburgh, Pittsburgh, Pennsylvania, U.S.A. 15. x ContributorsF. Bautista Departamento de Ingeniera Qumica, Universidad de Gua-dalajara,Guadalajara, MexicoVance Bergeron Ecole Normale Superieure, Paris, FranceArun Kumar Chattopadhyay Corporate Research and Development,United States Bronze Powders Group of Companies, Haskell, New Jersey,U.S.A.Arabinda Chaudhuri Division of Lipid Science and Technology, IndianInstitute of Chemical Technology, Hyderabad, IndiaSherril D. Christian Institute for Applied Surfactant Research, Univer-sityof Oklahoma, Norman, Oklahoma, U.S.A.PerM.Claesson Department of Chemistry, Surface Chemistry, Royal In-stituteof Technology, and Institute for Surface Chemistry, Stockholm, Swe-denJohn H. Clint Department of Chemistry, Hull University, Hull, UnitedKingdomL. Dahne Max-Planck-Institute of Colloids and Interfaces, Potsdam, Ger-manyPrasanta KumarDas* Division of Lipid Science and Technology, IndianInstitute of Chemical Technology, Hyderabad, IndiaF. Debuigne Laboratoire de Resonance Magnetique Nucleaire, FacultesUniversitaires Notre-Dame de la Paix, Namur, BelgiumImre Dekany Department of Colloid Chemistry, University of Szeged,Szeged, HungaryNikolai D. Denkov Laboratory of Chemical Physics and Engineering,Faculty of Chemistry, Sofia University, Sofia, BulgariaSurekha Devi Department of Chemistry, M. S. University of Baroda,Baroda, Gujarat, IndiaDeceased.*Current affiliation: Massachusetts Institute of Technology, Cambridge, Massachusetts, U.S.A. 16. Contributors xiEdwin Donath Department of Biophysics, Institute of Medical Physicsand Biophysics, University of Leipzig, Leipzig, GermanyWilliam A. Ducker Department of Chemistry, Virginia Tech, Blacksburg,Virginia, U.S.A.Julian Eastoe School of Chemistry, University of Bristol, Bristol, UnitedKingdomJordi Esquena Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, SpainShmaryahu Ezrahi Technology and Development Division, IDF, TelHashomer, IsraelAna Maria Forgiarini Departament de Tecnologia de Tensioactius, Insti-tutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, SpainChangyou Gao Department of Polymer Science and Engineering, Zhe-jiangUniversity, Hangzhou, Peoples Republic of ChinaAnilkumar G. Gaonkar Research and Development, Kraft Foods, Inc.,Glenview, Illinois, U.S.A.Mara Jose Garca-Celma Departament de Farma`cia, Facultad de Far-ma`cia, Universitat de Barcelona, Barcelona, SpainNissim Garti Casali Institute of Applied Chemistry, The Hebrew Univer-sityof Jerusalem, Jerusalem, IsraelBrian Grady School of Chemical Engineering and Materials Science,University of Oklahoma, Norman, Oklahoma, U.S.A.MichaelGradzielski Physical Chemistry I, University of Bayreuth, Bay-reuth,GermanyAsenD. Hadjiiski Laboratory of Chemical Physics and Engineering, Fac-ultyof Chemistry, Sofia University, Sofia, BulgariaMaria Hager Institute for Surface Chemistry, Stockholm, Sweden 17. xii ContributorsNorikatsu Hattori* Department of Applied Chemistry, Nagoya Instituteof Technology, Nagoya, JapanRichard K. Heenan ISIS Facility, Rutherford Appleton Laboratory, Chil-ton,United KingdomHeinz Hoffmann Physical Chemistry I, University of Bayreuth, Bay-reuth,GermanyKrister Holmberg Department of Applied Surface Chemistry, ChalmersUniversity of Technology, Goteborg, SwedenKlaus Horbaschek Physical Chemistry I, University of Bayreuth, Bay-reuth,GermanyQun Huo Department of Polymers and Coatings, North Dakota State Uni-versity,Fargo, North Dakota, U.S.A.Ivan B. Ivanov Laboratory of Chemical Physics and Engineering, Facultyof Chemistry, Sofia University, Sofia, BulgariaPaqui Izquierdo Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, SpainL. Jeunieau Laboratoire de Resonance Magnetique Nucleaire, FacultesUniversitaires Notre-Dame de la Paix, Namur, BelgiumEui-Chul Kang Department of Synthetic Chemistry and BiologicalChemistry, Kyoto University, Kyoto, JapanDae-Wook Kim Department of Chemical Engineering and CUPS, Han-yangUniversity, Seoul, KoreaJin-Chul Kim Department of Chemical and Biomolecular Engineering,KAIST, Daejeon, KoreaJong-Duk Kim Department of Chemical and Biomolecular Engineering,KAIST, Daejeon, KoreaDouglas T. Lai Development Center for Biotechnology, Taipei, Taiwan*Current affiliation: Chisso Corporation, Tokyo, Japan. 18. Contributors xiiiDominique Langevin Laboratoire de Physique des Solides, UniversiteParis Sud, Orsay, FranceRoger M. Leblanc Department of Chemistry, University of Miami, CoralGables, Florida, U.S.A.Eun-Ok Lee Department of Chemical and Biomolecular Engineering,KAIST, Daejeon, KoreaStefano Leporatti Institute of Medical Physics and Biophysics, Univer-sityof Leipzig, Leipzig, GermanyGanzuo Li Key Laboratory for Colloid and Interface Chemistry of StateEducation Ministry, Shandong University, Jinan, Peoples Republic of ChinaHeinz Lichtenfeld Max-Planck-Institute of Colloids and Interfaces, Pots-dam,GermanyJ. Lyklema Physical Chemistry and Colloid Science, Wageningen Uni-versity,Wageningen, The NetherlandsOctavio Manero Instituto de Investigaciones en Materiales, UniversidadNacional Autonoma de Mexico, Mexico City, MexicoC. Manohar Department of Chemical Engineering, Indian Institute ofTechnology, Mumbai, IndiaLaura Marquez Laboratory FIRP, School of Chemical Engineering, Uni-versityof the Andes, Merida, VenezuelaIsabel Mira* Laboratory FIRP, School of Chemical Engineering, Univer-sityof the Andes, Merida, VenezuelaHelmuth Mohwald Max-Planck-Institute of Colloids and Interfaces,Potsdam, GermanyDaniel Morales Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, Spain*Current affiliation: Institute for Surface Chemistry, Stockholm, Sweden. 19. xiv ContributorsBrij M. Moudgil Engineering Research Center, University of Florida,Gainesville, Florida, U.S.A.Sergio Moya Max-Planck-Institute of Colloids and Interfaces, Potsdam,GermanyJ. B.Nagy Laboratoire de Resonance Magnetique Nucleaire, FacultesUniversitaires Notre-Dame de la Paix, Namur, BelgiumCharmian J. OConnor Department of Chemistry, The University ofAuckland, Auckland, New ZealandSeong-Geun Oh Department of Chemical Engineering and CUPS, Han-yangUniversity, Seoul, KoreaHirofumi Okabayashi Department of Applied Chemistry, Nagoya Insti-tuteof Technology, Nagoya, JapanSzilvia Papp Department of Colloid Chemistry and Nanostructured Ma-terialsResearch Group, Hungarian Academy of Sciences, University of Sze-ged,Szeged, HungaryRita Patakfalvi Department of Colloid Chemistry and NanostructuredMaterials Research Group, Hungarian Academy of Sciences, University ofSzeged, Szeged, HungaryAlison Paul School of Chemistry, University of Bristol, Bristol, UnitedKingdomAlejandro Pen a* Laboratory FIRP, School of Chemical Engineering,University of the Andes, Merida, VenezuelaJeffrey Penfold ISIS Facility, Rutherford Appleton Laboratory, Chilton,United KingdomJ. H. Perez-Lopez Departamento de Ingeniera Qumica, Universidad deGuadalajara, Guadalajara, MexicoNaveen Kumar Pokhriyal Department of Chemistry, M. S. Universityof Baroda, Baroda, Gujarat, India*Current affiliation: Rice University, Houston, Texas, U.S.A. 20. Contributors xvJ. E. Puig Departamento de Ingeniera Qumica, Universidad de Gua-dalajara,Guadalajara, MexicoIgor Radtchenko Max-Planck-Institute of Colloids and Interfaces, Pots-dam,GermanyNalam Madhusudhana Rao Centre for Cellular and Molecular Biology,Hyderabad, IndiaI. Ravet-Bodart Laboratoire de Resonance Magnetique Nucleaire, Facul-tes Universitaires Notre-Dame de la Paix, Namur, BelgiumLaurence S. Romsted Department of Chemistry and Chemical Biology,Rutgers, The State University of New Jersey, New Brunswick, New Jersey,U.S.A.Emily Rumsey School of Chemical Sciences, University of East Anglia,Norwich, United KingdomJean-Louis Salager Laboratory FIRP, School of Chemical Engineering,University of the Andes, Merida, VenezuelaJohn F. Scamehorn Institute for Applied Surfactant Research, Univer-sityof Oklahoma, Norman, Oklahoma, U.S.A.Dinesh O. Shah Departments of Chemical Engineering and Anesthesi-ology,Center for Surface Science and Engineering, University of Florida,Gainesville, Florida, U.S.A.Qiang Shen Key Laboratory for Colloid and Interface Chemistry of StateEducation Ministry, Shandong University, Jinan, Peoples Republic of ChinaSeung-Il Shin Department of Chemical Engineering and CUPS, HanyangUniversity, Seoul, KoreaAndrew R. Slagle Institute for Applied Surfactant Research, Universityof Oklahoma, Norman, Oklahoma, U.S.A.Conxita Solans Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, SpainJ. F. A. Soltero Departamento de Ingeniera Qumica, Universidad deGuadalajara, Guadalajara, Mexico 21. xvi ContributorsGollapudi Venkata Srilakshmi Division of Lipid Science and Technol-ogy,Indian Institute of Chemical Technology, Hyderabad, IndiaDavid Steytler School of Chemical Sciences, University of East Anglia,Norwich, United KingdomG. B. Sukhorukov Max-Planck-Institute of Colloids and Interfaces, Pots-dam,GermanyCynthia Q. Sun* Department of Chemistry, The University of Auckland,Auckland, New ZealandJunzo Sunamoto Advanced Research and Technology Center, NiihamaNational College of Technology, Ehime, JapanSlavka S. Tcholakova Laboratory of Chemical Physics and Engineering,Faculty of Chemistry, Sofia University, Sofia, BulgariaEdward E. Tucker Institute for Applied Surfactant Research, Universityof Oklahoma, Norman, Oklahoma, U.S.A.Eric Tyrode Laboratory FIRP, School of Chemical Engineering, Univer-sityof the Andes, Merida, VenezuelaNu ria Uson Departament de Tecnologia de Tensioactius, InstitutdInvestigacions Qumiques i Ambientals de Barcelona, Barcelona, SpainA. Voight Max-Planck-Institute of Colloids and Interfaces, Potsdam, Ger-manyWolfgang von Rybinski Corporate Research, Henkel KGaA, Dussel-dorf,GermanyMatthiasWegener Corporate Research, Henkel KGaA, Dusseldorf, Ger-manyNoelia B. Zambrano Laboratory FIRP, School of Chemical Engineer-ing,University of the Andes, Merida, VenezuelaCurrent affiliation:*Hort Research, Auckland, New Zealand.Royal Institute of Technology (KTH), Stockholm, Sweden.M.W. Kellogg Ltd., Middlesex, United Kingdom. 22. Contributors xviiWeican Zhang State Key Laboratory of Microbial Technology, ShandongUniversity, Jinan, Peoples Republic of ChinaLi-Qiang Zheng Department of Chemistry, Shandong University, Jinan,Peoples Republic of ChinaLinda Zhu Institute for Applied Surfactant Research, University ofOklahoma, Norman, Oklahoma, U.S.A. 23. 11Highlights of Research onMolecular Interactions atInterfaces from the Universityof FloridaDINESH O. SHAH and BRIJ M. MOUDGIL University ofFlorida, Gainesville, Florida, U.S.A.ABSTRACTAn overview of research highlights of the past three decades from the Uni-versityof Florida on molecular interactions at interfaces and in micelles ispresented. This overview includes work on (1) the kinetic stability of mi-cellesin relation to technological processes, (2) molecular packing in mixedmonolayers and phase transition in monolayers, (3) microemulsions and theirtechnological applications including enhanced oil recovery (EOR) processesand preparation of nanoparticles of advanced materials, (4) adsorption ofpolymers at solidliquid interfaces and selective flocculation, and (5) themechanical strength of surfactant films at the solidliquid interface and itscorrelation with dispersion stability as well as the interfacial phenomena inchemicalmechanical polishing (CMP) of silicon wafers. Detailed resultsexplaining the role of molecular interactions at interfaces and in micelles aswell as pertinent references are given for each phenomenon discussed.I. INTRODUCTIONIt is a great pleasure and privilege for us to summarize the highlights ofresearch on molecular interactions at interfaces from the University ofFlorida on this 13th International Symposium on Surfactants in Solution(SIS-2000). During the past 30 years at the University of Florida, we havehad an ongoing research program on fundamental aspects as well as tech-nologicalapplications of interfacial processes. Specifically, this overview 24. 2 Shah and Moudgilincludes the kinetic stability of micelles in relation to technological pro-cesses,molecular packing in mixed monolayers and phase transition in mon-olayers,microemulsions and their technological applications including en-hancedoil recovery (EOR) processes and preparation of nanoparticles ofadvanced materials, adsorption of polymers at solidliquid interface andselective flocculation, the mechanical strength of surfactant films at thesolidliquid interface and dispersion stability, and the interfacial phenomenain chemical mechanical polishing (CMP) of silicon wafers.II. MONOLAYERSDuring the past quarter century, considerable studies have been carried outon the reactions in monomolecular films of surfactant, or monolayers. Figure1 shows the surface pressurearea curves for dioleoyl, soybean, egg, anddipalmitoyl lecithins [1]. For these four lecithins, the fatty acid compositionwas determined by gas chromatography. The dioleoyl lecithin has bothchains unsaturated, soybean lecithin has polyunsaturated fatty acid chains,egg lecithin has 50% saturated and 50% unsaturated chains, and dipalmitoyllecithin has both chains fully saturated. It is evident that, at any fixed surfacepressure, the area per molecule is in the following order:Dioleoyl lecithinsoybean lecithinegg lecithindipalmitoyl lecithinIt can be assumed that the area per molecule represents the area of a squareat the interface. Thus, the square root of the area per molecule gives thelength of one side of the square, which represents the intermolecular dis-tance.Figure 2 schematically illustrates the area per molecule and inter-moleculardistance in these four lecithins. The corresponding intermoleculardistances were calculated to be 9.5, 8.8, 7.1, and 6.5 A, respectively, at asurface pressure of 20 mN/m [2]. Thus, one can conclude that a change inthe saturation of the fatty acid chains produces subangstrom changes in theintermolecular distance in the monolayer.In addition, it was desired to explore the effects that these small changesin intermolecular distance had on the enzymatic susceptibility of these lec-ithinsto hydrolytic enzymes such as phospholipase A [35], a potent hy-drolyticenzyme found in cobra venom. Thus, microgram quantities of en-zymewere injected under this monolayer. By measuring the rate of changeof surface potential, one can indirectly measure the rate of reaction in themonolayer. It is assumed that these quantities [i.e., change in surface poten-tial(V) and the extent of reaction] are proportional to each other. Thekinetics of hydrolysis, as measured by a decrease in surface potential, werestudied for each lecithin monolayer as a function of initial surface pressureand are shown in Fig. 3 [6]. It was found that initially the reaction rate 25. Molecular Interactions at Interfaces 3FIG. 1 Surface pressurearea curves of dipalmitoyl, egg, soybean, and dioleoyllecithins.increased as the surface pressure increased. Subsequently, as the surfacepressure increased further, the reaction rate decreased until a critical surfacepressure was reached at which no reaction occurred. The critical surfacepressure required to block the hydrolysis of lecithin monolayer increasedwith the degree of unsaturation of fatty acid chains (Fig. 3). Thus, it appearsthat as the intermolecular distance increases because of the unsaturated fattyacid chains, a higher surface pressure is required to clock the penetration ofthe active site of the enzyme into the monolayer to cause hydrolysis. Thisalso led to a suggestion that subangstrom changes in the intermoleculardistance in the monolayer were significant for the enzymatic hydrolysis ofthe monolayers. 26. 4 Shah and MoudgilFIG. 2 Schematic representation of the area per molecule and intermolecular dis-tancein dioleoyl, soybean, egg, and dipalmitoyl lecithin monolayers based on thedata plotted in Fig. 1.In addition to hydrolysis reactions, the enzymatic synthesis in monolayerswas studied [7]. In this case, a steric acid monolayer was formed on anaqueous solution containing glycerol. After compression to a desired surfacepressure, a small amount of enzyme lipase was injected under the monolayer.The lipase facilitated the linkage of glycerol with fatty acid and producedmonoglycerides, diglycerides, and triglycerides in the monolayer [810],which could be detected by thin-layer chromatography (TLC) or high-per-formanceliquid chromatography (HPLC).Because the amount of product that can be synthesized using a monolayeris in microgram quantities, this method is not attractive for large-scale en-zymaticsynthesis. Therefore, studies of enzymatic reactions in monolayerswere extended to studies of enzymatic reactions in a foam. A foam providesa large interfacial area, and by continuous aeration one can generate an evenlarger interfacial area. A soap bubble is stabilized by monolayers on bothinside and outside surfaces of the bubble (Fig. 4). The glycerol and enzymecan be added into the aqueous phase before producing the foam. Thus, itwas shown that almost 88% of free steric acid could be converted to di- andtriglycerides in 2 h by reactions in foams (Fig. 5). For surface-active sub-strates(or reactants) and enzymes, reactions in foams offer a very interestingpossibility to produce large-scale synthesis of biochemicals using a foam asa reactor. 27. Molecular Interactions at Interfaces 5FIG. 3 Hydrolysis rate (as measured by surface potential) versus initial surfacepressure of various lecithin monolayers.Another interesting investigation at the Center for Surface Science andEngineering (CSSE) was focused on the possible existence of phase tran-sitionsin mixed monolayers of surfactants. Figure 6 shows the rate of evap-orationfrom pure and mixed monolayers of cholesterol and arachidyl (C20)alcohol, as well as their mixed monolayers [11]. It is evident that the pure 28. 6 Shah and MoudgilFIG. 4 Schematic diagram of a lipase-catalyzed reaction in a foam vessel.FIG. 5 Decrease in free fatty acids and synthesis of di- and triglycerides by lipasein foam. 29. Molecular Interactions at Interfaces 7FIG. 6 Rate of evaporation from pure and mixed monolayers of cholesterol andarachidyl (C20) alcohol.C20 alcohol monolayer allows only one third of the water loss related toevaporation of the pure cholesterol monolayer. This is presumably due tothe fact that the C20 alcohol forms monolayers that are in the two-dimen-sionalsolid state. In contrast, the cholesterol monolayers are in the two-dimensionalliquid state. However, when cholesterol is incorporated into aC20 alcohol monolayer, the cholesterol mole fraction needs to be only about20% to liquefy the solid monolayers of C20 alcohol. The abrupt increase inevaporation rate of water at 2025 mol% cholesterol illustrates the two- 30. 8 Shah and Moudgildimensional phase transition in the mixed monolayers from a solid state toa liquid state. After the cholesterol fraction reaches about 25 mol%, themonolayer remains in the two-dimensional liquid state and, hence, there isno further change in the rate of evaporation of water. Thus, one can utilizethe evaporation of water through a film as a very sensitive probe for ob-servingthe molecular packing in monolayers. The existence of a solid stateor a liquid state for monolayers can be inferred from such experimentalresults.It has been shown that mixed monolayers of oleic acid and cholesterolexhibit the minimum rate of evaporation at a 1:3 molar ratio of oleic acidto cholesterol. This is shown in mixed monolayers of oleic acid and cho-lesterolin Fig. 7 as a function of surface pressure [11]. In has further beenshown that a 1:3 molar ratio in mixed fatty acid and fatty alcohol monolay-ers,one observes the maximum foam stability, minimum rate of evaporation,and maximum surface viscosity in these systems [12].Monolayers are fascinating systems with extreme simplicity and well-definedparameters. During the past 35 years of research, we have found thestudies on monolayers to be rewarding in understanding the phenomenaoccurring at the gasliquid, liquidliquid, and solidliquid interfaces inrelation to foams, emulsions, lubrication, and wetting processes.III. MICELLE KINETICS ANDTECHNOLOGICAL APPLICATIONSIt is well recognized that a surfactant solution has three components: sur-factantmonomers in the aqueous solution, micellar aggregates in solution,and monomers absorbed as a film at the interface. The surfactant is in dy-namicequilibrium among all of these components. From various theoreticalconsiderations as well as experimental results, it can be assumed that mi-cellesare dynamic structures whose stability is in the range of millisecondsto seconds. Thus, in an aqueous surfactant solution, micelles break and re-format a fairly rapid rate [1315]. Figure 8 shows the two characteristicrelaxation times, 1 and 2, associated with micellar solutions. The shortertime, 1, generally of the order of microseconds, is related to the exchangeof surfactant monomers between the bulk solution and the micelles, whereasthe longer time, 2, generally of the order of milliseconds to seconds, isrelated to the formation or dissolution of a micelle after several molecularexchanges [13,14]. It has been proposed that the lifetime of a micelle canbe approximated by n2, where n is the aggregation number of the micelle[15]. Thus, relaxation time 2 is proportional to the lifetime of the micelle.A large value of 2 represents high stability of the micellar structure. 31. Molecular Interactions at Interfaces 9FIG. 7 Minimum rate of evaporation at a 1:3 molar ratio in mixed monolayers ofoleic acid and cholesterol.Figure 9 shows the relaxation time 2 of micelles of sodium dodecylsulfate (SDS) as a function of SDS concentration [13,16,17]. It is evidentthat the maximum relaxation time of micelles is observed at an SDS con-centrationof 200 mM. This implies that SDS micelles are most stable atthis concentration. For several years researchers at the CSSE have tried tocorrelate the measured 2 with various equilibrium properties such as surfacetension, surface viscosity, and others, but no correlation could be found.However, a strong correlation of 2 with various dynamic processes such asfoaming ability, wetting time of textiles, bubble volume, emulsion dropletsize, and solubilization of benzene in micellar solution was found [18]. 32. 10 Shah and MoudgilFIG. 8 Two relaxation times of micelles, 1 and 2, and related molecular processes.Figures 10 and 11 summarize the effects of SDS concentration on thephenomena mentioned as well as on other related phenomena. Figure 10shows typical phenomena in liquidgas systems, and Fig. 11 shows typicalphenomena in liquidliquid and solidliquid systems. It is evident that eachof these phenomena exhibits a maximum or minimum at 200 mM SDS,depending on the molecular process involved. Thus the take-home mes-sageemerging from our extensive studies of the past decades is that mi-cellarstability can be the rate-controlling factor in the performance of var-ioustechnological processes such as foaming, emulsification, wetting,bubbling, and solubilization [19].FIG. 9 Relaxation time, 2, of SDS micelles as a function of SDS concentration.Maximum 2 found at 200 mM (vertical line). 33. Molecular Interactions at Interfaces 11FIG. 10 Various liquidgas system phenomena exhibiting minima or maxima at200 mM SDS.The currently accepted explanation for the effect of surfactant concentra-tionon micellar stability was proposed by Aniansson and coworkers in the1970s and expanded by Kahlweit and coworkers in the early 1980s [1316]. Anniansons model [1315] nicely predicts micelle kinetics at a lowsurfactant concentration based on stepwise association of surfactant mono-mers.Hence, the major parameters in this model are the critical micelleconcentration (cmc) and the total concentration of the surfactant in solution.At higher surfactant (and hence counterion) concentrations, experimentalresults begin to deviate from Anianssons model. Kahlweits fusionfissionmodel [16] takes into account the concentration and ionic strength of thecounterions in these solutions and proposes that as the counterion concen-trationincreases, the charge-induced repulsion between micelles and sub-micellaraggregates decreases, leading to coagulation of these submicellaraggregates.In both of these models, the effects of intermicellar distance as well asthe distance between submicellar aggregates have not been taken into ac- 34. 12 Shah and MoudgilFIG. 11 Various liquidliquid and solidliquid system phenomena exhibiting min-imaor maxima at 200 mM SDS.count. Researchers at the CSSE have been attempting to introduce the effectof intermicellar distance into micellar kinetic theory. As the SDS concentra-tionincreases, the number of micelles increases, and thus the intermicellardistance decreases. By knowing the aggregation number of the micelles, thenumber of micelles present in the solution can be calculated. The solutioncan then be divided into cubes such that each cube contains one micelle.From this, the distance between the centers of the individual cubes can betaken as the intermicellar distance.Researchers at the CSSE determined that the concentration at which theSDS micelle was most stable (200 mM) coincided with an intermicellardistance of approximately one micellar diameter [1923]. At this concen-tration,one would expect a tremendous coulombic repulsion between themicelles at such a short distance. A possible explanation for the observedstability is that there is a rapid uptake of sodium as a counterion on themicellar surface at this concentration, making the micelles more stable. Thus,the coulombic repulsion between micelles with the concomitant uptake of 35. Molecular Interactions at Interfaces 13sodium ions allows the stabilization of micelles at this intermicellar distanceand, hence, maximum 2 at 200 mM concentration.By introducing this so-called intermicellar coulombic repulsion model(ICRM) into existing theoretical models, better agreement between theoret-icallycalculated and experimental 2 values may be attained.It should be mentioned that Per Ekwall proposed first, second, and thirdcritical micelle concentrations for sodium octanoate solutions [22]. At thesecond cmc, he showed sudden uptake or binding of sodium ions to themicellar surface and he proposed that at the second cmc there was tightpacking of surfactant molecules in the micelle. Thus, our 200 mM SDSconcentration could be equivalent to the second cmc as proposed by Ekwall.The phenomenon of a surfactant exhibiting a maximum 2 appears to bea general behavior, and perhaps other anionic or cationic surfactants mayform tightly packed micelles at their own characteristic concentrations. Aswith the first cmc, this critical concentration may also depend upon physicaland chemical conditions such as temperature, pressure, pH, salt concentra-tion,and other parameters in addition to the molecular structure of the sur-factant[24,25]. Work is currently in progress at the CSSE on identifying asimilar critical concentration for nonionic surfactants as well as mixed sur-factantsystems.In addition to this work, it has been shown that upon incorporation of ashort-chain alcohol such as hexanol into the SDS micelles, the maximum 2occurs at a lower SDS concentration [2628]. Thus, it appears that in amixed surfactant system, one can produce the most stable micelle at a lowersurfactant concentration upon incorporation of an appropriate cosurfactant[29]. Also investigated was the effect of long-chain alcohols on the micellarstability. Results were similar to those for short-chain alcohols for all butdodecanol, which showed a significant increase in micellar stability overmicelles containing only SDS because of the chain length compatibility ef-fect[30].Figure 12 shows the effect of coulombic attraction between oppositelycharged polar groups as well as the chain length compatibility effect on the2 or micellar stability of SDS plus alkyltrimethylammonium bromide (cat-ionicsurfactant) solutions. It shows that surface tension, surface viscosity,miscellar stability, foaming ability, and foam stability are all influenced bythe coulombic interaction as well as the chain length compatibility effect[30,31]. It should be noted that the ratio (weight basis) of SDS to alkyltri-methylammoniumbromide was 95:5 in this mixed surfactant system. How-ever,even at this low concentration, the oppositely charged surfactant dra-maticallychanged the molecular packing of the resulting micelles as wellas the surfactant film absorbed at the interface. 36. 14 Shah and Moudgil 37. Molecular Interactions at Interfaces 15In view of the previous discussion, it is evident that micellar stability isof considerable importance to technological processes such as foaming,emulsification, wetting, solubilization, and detergency because a finely tuneddetergent formulation can significantly improve the cleaning efficiency aswell as reduce the washing time in a laundry machine, resulting in significantenergy savings at a national and global level. Micellar stability is thus acritical issue in any application in which surfactants are present as micelles,and the subsequent monomer flux is utilized in the application.IV. MICROEMULSIONS IN ENHANCED OIL RECOVERYAND SYNTHESIS OF NANOPARTICLESAs early as 1943, Professor J. H. Schulman published reports on transparentemulsions [32]. From various experimental observations and intuitive rea-soning,he concluded that such transparent systems were microemulsions.Figure 13 illustrates the transparent nature of a microemulsion in comparisonwith a macroemulsion. He also proposed the concept of a transient negativeinterfacial tension to induce the spontaneous emulsification in such systems.Considerable studies have been carried out on microemulsions during thepast quarter-century, during which time it has been recognized that there arethree types of microemulsions: lower-phase, middle-phase, and upper-phasemicroemulsions. The lower-phase microemulsion can remain in equilibriumwith excess oil in the system, the upper-phase microemulsion can remain inequilibrium with excess water, and the middle-phase microemulsion can re-mainin equilibrium with both excess oil and water. As a result, the lower-phasemicroemulsion has been considered to be an oil-in-water microemul-sion,the upper-phase microemulsion has been considered to be a water-in-oilmicroemulsion, whereas the middle-phase microemulsion has been the sub-jectof much research and has been proposed to be composed of bicontinuousor phase-separated swollen micelles from the aqueous phase [3344]. Figure14 shows the lower-, middle-, and upper-phase microemulsions, as repre-sentedby the darker liquid in each tube [4548].The formation of lower-, middle-, and upper-phase microemulsions isrelated to the migration of surfactant from lower phase to middle phase toupper phase. Figure 15 illustrates that migration of the surfactant from theFIG. 12 Effect of coulombic attraction between polar groups of surfactants andchain length compatibility on 2 or micellar stability and other interfacial propertiesof mixed surfactant solutions. The dashed line represents the specific property of100 mM pure SDS solution. 38. 16 Shah and MoudgilFIG. 13 Illustration of the transparent nature of a microemulsion compared with amacroemulsion.aqueous phase to the middle phase to the oil phase can be induced bychanging a number of parameters, including adding salts (e.g., NaCl) to thesystem, decreasing the oil chain length, increasing the surfactant molecularweight, adding a cosurfactant, and decreasing the temperature [48,49]. How-ever,it has been reported that in oil/water/nonionic surfactant systems, sur-factantmoves from lower phase to middle phase to upper phase as thetemperature is increased [42,43], so each system must be carefully analyzedin order to determine the effects of certain parameters.Microemulsions exhibit ultralow interfacial tension with excess oil orwater phases. Therefore, the middle-phase microemulsion is of special im-portanceto the process of oil displacement from petroleum reservoirs.A. Microemulsions in Enhanced Oil RecoveryFigure 16 shows a schematic view of a petroleum reservoir as well as theprocess of water or chemical flooding by an inverted five-spot pattern [33].Several thousand feet below the ground, oil is found in tightly packed sandor sandstones in the presence of water as well as natural gas. During theprimary and secondary recovery processes (water injection method), about35% of the available oil is recovered. Hence, approximately 65% is left inthe petroleum reservoir. This oil remains trapped because of the high inter-facialtension (2025 mN/m) between the crude oil and reservoir brine. Itis known that if the interfacial tension between crude oil and brine can bereduced to around 103 mN/m, one can mobilize a substantial fraction of 39. Molecular Interactions at Interfaces 17FIG. 14 Samples of (a) lower-, (b) middle-, and (c) upper-phase microemulsionsin equilibrium with excess oil, excess water and oil, or excess water, respectively.the residual oil in the porous media in which it is trapped. Once mobilizedby an ultralow interfacial tension, the oil ganglia must coalesce to form acontinuous oil bank. The coalescence of oil droplets has been shown to beenhanced by a very low interfacial viscosity in the system. The incorporationof these two critical factors into a suitable surfactant system for oil recoverywas crucial in developing the surfactantpolymer flooding process for en-hancedoil recovery from petroleum reservoirs. Conceptually, one injects asurfactant formulation in the porous media in the petroleum reservoir so thatupon mixing with the reservoir brine and oil it produces the middle-phasemicroemulsion in situ. This middle-phase microemulsion, which is in equi-libriumwith excess oil and excess brine, propagates throughout the petro-leumreservoir. The design of the process is such that the oil bank maintains 40. 18 Shah and MoudgilFIG. 15 The transition from lower- to middle- to upper-phase microemulsions canbe brought about by the addition of salts or by varying other parameters. The tran-sitionfrom lower to middle to upper phase (I m u) occurs by (1) increasingsalinity, (2) decreasing oil chain length, (3) increasing alcohol concentration (C4, C5,C6), (4) decreasing temperature (for ionic surfactants), (5) increasing total surfactantconcentration (for high-molecular-weight anionic surfactants), (6) increasing brine/oil ratio (for high-molecular-weight anionic surfactants), (7) increasing surfactantsolution/oil ratio (for high-molecular-weight anionic surfactants), and (8) increasingmolecular weight of surfactant.ultralow interfacial tension with reservoir brine until it arrives at the pro-ductionwells.One parameter that has been discovered to be crucially important in thesuccessful implementation of the surfactantpolymer flooding process is thesalinity of the aqueous phase. As discussed previously, addition of salt tothe microemulsion system induces the change from lower- to middle- toupper-phase microemulsion (Fig. 15) [33]. It was found that at a particularsalt concentration, deemed the optimal salinity, a number of important pa-rameterswere optimized for the oil recovery process. The optimal salinitywas found to occur when equal amounts of oil and brine were solubilizedby the middle-phase microemulsion [50].Figure 17 summarizes the various parameters that are important in thesurfactantpolymer flooding process as a function of salt concentration[33,5154]. It is evident that all of these parameters exhibit a maximum ora minimum at the optimal salinity. Thus, it appears that all of these processesare interrelated for the oil displacement in porous media by the surfactantpolymer flooding process. It also appears that the optimal salinity value isa crucial parameter for consideration of a system to be used in this process.B. Formation of Nanoparticles Using MicroemulsionsAnother very interesting use of microemulsions that has been investigatedin our laboratory over the past decade is in the production of nanoparticles. 41. Molecular Interactions at Interfaces 19FIG. 16 Schematic view of a petroleum reservoir and the process of water or chem-icalflooding (five-spot pattern).Figure 18 schematically illustrates the formation of nanoparticles using wa-ter-in-oil microemulsions. For this process, two identical water-in-oil mi-croemulsionsare produced, the only difference between the microemulsionsbeing the nature of the aqueous phase, into which the two water-solublereactants, A and B, are dissolved separately. Upon mixing the two nearlyidentical microemulsions, the water droplets collide and coalesce, allowingthe mixing of the reactants to produce the precipitate AB. Ultimately, thesedroplets again disintegrate into two aqueous droplets, one containing thenanoparticle AB and the other containing just the aqueous phase [5557].Thus, a precipitation reaction can be carried out in the aqueous cores ofwater-in-oil microemulsions using the dispersed water droplets as nanoreac-tors.The size of the particles formed is physically limited by the reactantconcentration as well as the size of the water droplets. In this way, 42. 20 Shah and MoudgilFIG. 17 Various phenomena occurring at the optimal salinity in the surfactantpolymer flooding process for enhanced oil recovery.monodisperse particles in the range 210 nm in diameter can be produced.The production of nanoparticles with homogeneity of particle size (i.e., smallsize range) has inherently been a problem with other conventional methods.This method of nanoparticle synthesis is an improvement over other methodsfor applications that require the production of monodisperse nanoparticles[5860].Superconducting nanoparticles have also been produced in our laboratoryusing the microemulsion method. Table 1 shows the composition of the twomicroemulsions used for synthesizing nanoparticles of YBCO (Yttrium Bar- 43. Molecular Interactions at Interfaces 21FIG. 18 Formation of nanoparticles using microemulsions (water-in-oil) as nano-reactors.The water droplets continually collide, coalesce, and break up upon mixingof two microemulsions containing reactants.ium Copper Oxide) superconductor [6163]. In this case, water-soluble saltsof yttrium, barium, and copper were dissolved in the aqueous cores of onemicroemulsion and ammonium oxalate was dissolved in the aqueous coresof the other microemulsion. Upon mixing the two microemulsions, precursornanoparticles of metal oxalates were formed. The nanoparticles were cen-trifugedand then washed with chloroform, methanol, or acetone to removethe surfactants and oil.These nanopowders were then calcined at the appropriate temperature toconvert the oxalate precursors into oxides of these materials. The oxideswere then compressed into a pellet and sintered at 860C for 24 h. The pelletwas cooled, and the critical temperature of zero resistance was measured. Itwas found that this critical temperature did not show any change from crit- 44. 22 Shah and MoudgilTABLE 1 Composition of Two Microemulsions for Synthesizing Nanoparticles ofYBCO SuperconductorsSurfactant phaseHydrocarbonphase Aqueous phaseMicroemulsion I CTAB1-butanol n-Octane (Y, Ba, Cu)nitratesolution, total metalconcentration = 0.3 NMicroemulsion II CTAB1-butanol n-Octane Ammonium oxalatesolution, 0.45 NWeight fraction (forboth I and II)29.25% 59.42% 11.33%ical temperatures of superconductors produced by the traditional coprecipi-tationmethod. However, the fraction of the ideal Meissner shielding wasstrikingly different for the two samples prepared by different methods. It isthe Meissner effect that is related to the levitational effect of the supercon-ductingpellet on a magnetic field. Thus, it appears that the leakage of mag-neticflux from the conventionally prepared sample was greater than thatfrom the sample produced by microemulsion-derived nanoparticles. Figure19 shows scanning electron microscope (SEM) images of sintered pelletsproduced by the two methods. It is evident that the pellets prepared fromthe nanoparticles produced by the microemulsion method showed 30100times larger grain size, less porosity, and higher density as compared withthe samples prepared by conventional precipitation of aqueous solutions ofthese salts. A possible explanation for these effects is that nanoparticles,because of their extremely small size and large surface area, can disintegratevery quickly and allow diffusion of atoms to the site of the growing grainsto support the growth process of the grains. Therefore, samples preparedfrom nanoparticles exhibit large grain size and low porosity [64]. Thus, itappears that these nanoparticles may be useful to produce high-density ce-ramics.Since the pioneering work of the first 20 years on the formation of nano-particlesof heavy metals by the microemulsion method [65], we have addedto the understanding of the mechanism and control of the reaction kineticsin microemulsions by controlling the interfacial rigidity of the microemul-siondroplet. We have introduced the concept of a chain length compatibilityeffect observed for the reactions in microemulsions, in which the interfacialrigidity is maximized by a certain chain length combination of the surfactant,oil, and cosurfactant alcohol, causing a decreased reaction rate [66]. All of 45. Molecular Interactions at Interfaces 23FIG. 19 SEM images of superconducting pellets prepared from nanopowders (aand b) using microemulsions and conventionally prepared powders (c and d) (byprecipitation of aqueous solutions).these contributions have led to a greater understanding of the method ofnanoparticle production using microemulsion media.V. CONTROL OF POLYMER ADSORPTION ATPARTICLE SURFACESPolymers at particle surfaces play an important role in a range of technol-ogiessuch as paints, polishing, filtration, separations, enhanced oil recovery,and lubrication. In order to optimize these technologies, it is important tounderstand and control the adsorption, conformation, and role of surfacemolecular architecture in selective polymer adsorption.For a particular polymer functionality, the adsorption depends on the na-tureand energetics of the adsorption sites that are present on the surface.The adsorption of polymers via electrostatics, chemical bonding, and hydro-phobicinteraction is relatively well understood, and most of the unexpectedadsorption behavior is attributed to hydrogen bonding, which is ubiquitous 46. 24 Shah and Moudgilin nature. Research carried out at the University of Florida EngineeringResearch Center for Particle Science and Technology (UF-ERC) has focusedon the role of surface chemistry and surface molecular architecture in hy-drogenbonding of polymers to particle surfaces. In addition, practical ap-plicationsof controlled polymer adsorption have been investigated forsolidsolid separations via selective flocculation technology.A. Control of Hydrogen BondingThe surfaces of most oxides and minerals have two different kinds of acidsites, Bronsted and Lewis, on which hydrogen-bonding polymers can adsorb.Bronsted acids are defined as proton donors, such as the MOH sites onoxide surfaces. The more electron withdrawing the underlying substrate, thegreater is the Bronsted acidity. Lewis acid sites are defined as electron de-ficientor as having the ability to accept electrons. Examples of Lewis acidsites include M(OH)2 groups on oxide surfaces.For a hydrogen-bonding polymer such as poly(ethylene oxide) (PEO),whose ether oxygen linkage acts as a Lewis base, it was illustrated that notonly did the number of hydrogen-bonding sites differ from one substrate toanother but also the energy of the hydrogen-bonding sites varied [67]. Basedon adsorption studies of PEO on silica and other oxides [67], it was deter-minedthat the amount of adsorbed polymer depended on the nature of theBronsted acid (proton donor) sites on the particles. Hence, the more electronwithdrawing the underlying substrate, the greater the Bronsted acidity, andthus the lower the point of zero charge (pzc) of the material. It is seen fromFig. 20 that SiO2, MoO3, and V2O5 strongly adsorb PEO whereas oxideswith pzc greater than that of silica, such as TiO2, Fe2O3, Al2O3, and MgO,did not exhibit significant adsorption of PEO. However, within a singlesystem (silica) it has been found that PEO will adsorb onto sol-gelderivedsilica but not onto glass or quartz at a pH of 9.5. This suggests that thestrength of Bronsted acid sites (higher ability to donate protons), as deter-minedby the surface molecular architecture, also influences the adsorptionprocess. It was also shown that the nature and energetics of the surface sitescould be modified by surface modification techniques such as calcinationand rehydroxylation [68], Upon calcination of a silica surface to 800C, thenumber of isolated surface hydroxyl groups [determined from Fourier trans-forminfrared (FTIR) spectroscopy] and three-membered silicate rings (de-terminedfrom Raman spectroscopy) increased, resulting in higher surfaceacidity [determined from solid-state nuclear magnetic resonance (NMR)spectroscopy using triethyl phosphine oxide probe]. These changes led tohigher adsorption of the PEO polymer (Fig. 21) [68].Based on the polymer functionality, it may be possible to predict thesurface molecular architecture or the surface sites that are required for the 47. Molecular Interactions at Interfaces 25FIG. 20 Effect of surface Bronsted acidity on the adsorption of a hydrogen-bondingpolymer, poly(ethylene oxide) (PEO). (From Ref. 31.)FIG. 21 Adsorption of PEO on sol-gel silica with different treatments (calcined800C, rehydroxylated). (From Ref. 32.) 48. 26 Shah and Moudgilpolymer to adsorb. Studies of the adsorption of PEO, PAM (nonionic poly-acrylamide),PAA [nonionic poly(acrylic acid)], PAH (polyallylamine hy-drochloride),and PVA (poly (vinyl alcohol)) onto silica, alumina, and he-matitewere carried out, and a master table correlating functional groupactive surface site relationships was developed (Table 2) [69]. Strongadsorption of PEO onto silica was observed, with none onto alumina andhematite, in agreement with the earlier results [67].No adsorption of PAM onto silica was observed; however, PAM wasfound to adsorb onto hematite and alumina at pH 3.0. At pH 9.5, there wasno adsorption of PAM onto alumina. Given the lack of adsorption of PAMonto silica, which has strong Bronsted acid sites, it was concluded that PAMTABLE 2 Correlation of Polymer Functionality with the Surface Adsorption SitesPolymer Repeat unit FunctionalityAdsorbsontoAdsorptionsitesPEO Ether SiO2 BronstedPVA Hydroxyl SiO2 BronstedPAA Carboxylic acid Fe2O3Al2O3TiO2LewisPAM Amide Fe2O3Al2O3TiO2LewisPAH Amine SiO2 BronstedPEO, poly(ethylene oxide); PVA, poly(vinyl alcohol); PAA, poly(acrylic acid); PAM, poly-(acrylamide) (nonionic); PAH, polyallylamine.Source: Ref. 33. 49. Molecular Interactions at Interfaces 27adsorbed on Lewis acid sites. The lack of adsorption of PAM onto aluminaat pH 9.5 was explained by poisoning of active Lewis sites due to pref-erentialadsorption of hydroxide ions at pH values below the isoelectric pointof alumina (pHiep = 8.8).Nonionic PAA was found to adsorb onto hematite and alumina but notonto silica at pH 3. Adsorption experiments were not conducted above pH3 because PAA becomes ionized and the adsorption is dominated by elec-trostaticinteractions.Having developed the fundamental knowledge base on the role of sur-facemolecular architecture in polymer adsorption and the surface sitepoly-merfunctional group correlation, the next logical step was to use thesefundamentals in real particulate systems, which contain heterogeneoussurfaces with impurities that in some cases may foil selective adsorptionschemes.B. Control of Polymer Adsorption forSelective SeparationFlocculation of fine particles using polymeric materials (flocculants) andseparation of such aggregates from particles of the other component(s) inthe dispersed phase is known as selective flocculation [67]. The competitionbetween different surfaces for the flocculant must be controlled in order toachieve adsorption on the targeted component(s). The aggregates of the poly-mer-coated particles, or flocs, thus formed are separated from the suspen-sionby either sedimentationelutriation or floc flotation.The major barrier to further commercialization of the selective floccula-tiontechnology is the poor success in extension of single-component suc-cessesto mixed-component systems. The selectivity observed in single-com-ponenttests is often lost in mixed-component or natural systems. One ofthe significant reasons for this loss in selectivity is heteroflocculation,wherein a small amount of polymer adsorption on the inert material leadsto coflocculation with the active material. One of the major advances at theUniversity of Florida (UF) has been the development of the site-blockingagent (SBA) concept [70,71] to overcome heteroflocculation, thus achievingselectivity in particle separation. The SBA concept is illustrated schemati-callyin Fig. 22. The concept involves blocking all the active sites for poly-meradsorption on the inert material (component of the particle mixture notto be flocculated) by addition of the SBA. After the addition of the SBA,when the flocculant is added, it adsorbs only onto the active component(material intended to be flocculated), resulting in selectivity of separation.A lower molecular weight fraction of the same or a similar flocculant, whichon its own is incapable of inducing flocculation in the active or the floc- 50. 28 Shah and MoudgilFIG. 22 Schematic illustrating the site-blocking agent (SBA) concept. (FromRef. 31.)culating material, was successfully used as an SBA to minimize heterofloc-culation.The concept has since been commercialized by Engelhard Corpo-rationfor removal of titania impurities from kaolin clays [72].VI. SURFACTANT SELF-ASSEMBLY AT THESOLIDLIQUID INTERFACESurfactants at the solidliquid interface are used in various industrial pro-cessesranging from ore flotation and paint technology to enhanced oil re-covery[73]. Apart from the traditional uses of surfactants, surfactant struc-turesare increasingly being investigated as organic templates to synthesizemesoscopic inorganic materials with controlled nanoscale porosity, whichare expected to have applications in electronics, optics, magnetism, and ca-talysis[74]. Surfactant structures at the solidliquid interface have also beenutilized to stabilize particulate dispersions [7577]. Recent work carried outat UR-ERC has shown that self-assembled surfactants can be utilized to 51. Molecular Interactions at Interfaces 29prepare particulate dispersions under extreme conditions [78,79] in whichtraditionally used dispersing methods, such as electrostatics (surface charge),inorganic dispersants (sodium silicate, sodium hexametaphosphate), andpolymers, may not result in a completely dispersed suspension.Figure 23 depicts both the suspension turbiditya measure of stabilityand the surface forces present between the atomic force microscope(AFM) tip and silica substrate as a function of dodecyltrimethylammoniumbromide (C12TAB) concentration at pH 4 and 0.1 M NaCl. Under theseconditions, the silica suspension without a dispersant is unstable. As thesurfactant concentration is increased, the suspension remains unstable untila surfactant concentration of about 8 mM. Between surfactant concentrationsof 8 and 10 mM, a sharp transition in the stability (unstable to stable) andforces (no repulsion to repulsion) is observed. A good correlation existsbetween the suspension stability and repulsive forces due to self-assembledsurfactant aggregates. The repulsive force is an order of magnitude higherthan electrostatic forces alone, indicating that the repulsion is steric in origin.It was proposed that the dominant repulsion mechanism was the steric re-pulsiondue to the elastic deformation of the self-assembled aggregates whentwo surfaces approached each other.The adsorption, zeta potential, and contact angle measurements on silicasurfaces in 0.1 M NaCl at pH 4.0 as a function of solution C12TAB concen-FIG. 23 Turbidity of silica particles after 60 min in a solution of 0.1 M NaCl atpH 4 as a function of C12TAB concentration, and the measured interaction forcesbetween an AFM tip and silica substrate under identical solution conditions. (FromRef. 42.) 52. 30 Shah and Moudgiltration were measured and are presented in Fig. 24. Based on interface prop-erties,the entire self-assembly process was divided into six stages, markedAF in Fig. 24. At low concentrations (below 0.007 mM, region A in Fig.24), individual surfactant adsorption takes place. The next structural transi-tionin this system was the formation of hemimicelles, which is evidencedby a significant effect on both the zeta potential and hydrophobicity of thesurface. At approximately 0.1 mM in region B, the sign of the zeta potentialreverses but the contact angle continues to increase, indicating that the re-versalin zeta potential is not due to formation of bilayers as suggested inthe past [73,7577]. This reversal in zeta potential while the hydrophobicitycontinues to increase was attributed either to hydrophobic association be-tweenthe surfactant tails, resulting in formation of hemimicelles, or to somekind of specific adsorption.In regions B and C, contact angle continues to increase, indicating in-creasingconcentration of hemimicelles at the interface. Beyond a certainconcentration (approximately 2.3 mM), in region D the hydrophobicity de-creases,accompanied by a sharp increase in the zeta potential. This indicatesthe formation of structures with an increasing number of polar heads orientedtoward the solution. The sharp increase in zeta potential and a correspondingdecrease in contact angle were attributed to the transition of surfactant struc-turefrom hemimicelles to either bilayers, spherical aggregates (imaged atsurfaces using AFM), or structures having semispheres on top of perfectmonolayers (compact monolayer covering the entire surface), as suggestedby Johnson and Nagarajan [80]. At higher surfactant concentrations beyondFIG. 24 Adsorption isotherm (squares), zeta potential (triangles), and contact angle(spheres) of silica surfaces in 0.1 M NaCl at pH 4.0 as a function of solution C12TABconcentration. (From Refs. 42 and 43.) 53. Molecular Interactions at Interfaces 31the bulk cmc (regions E and F), based on AFM imaging, spherical aggregatesor composite semispheres on top of perfect monolayers (it is not possibleto distinguish between these structures on the basis of just AFM imaging)are known to exist [81]. Thus, based on the adsorption, zeta potential, andcontact angle results, several plausible surfactant structures were proposedat the interface at different concentrations of the surfactant. To illustrate theexact structural transitions taking place at the interface, the FTIRATR (withpolarized IR beam) technique, which can probe the adsorbed structures di-rectly,was employed.The FTIRATR technique relies on the fact that individual surfactantmolecules, hemimicelles, monolayers, bilayers, and spherical or cylindricalaggregates at the interface will have different average orientations of thealkyl chains with respect to the surface normal. Different average orienta-tionsresult in different absorptions of the plane-polarized IR beam and canthus be used to identify the surfactant structures at the interface [79].Based on the FTIRATR study, the proposed surfactant structures in thedifferent regions were verified. In region D, it was found that sphericalaggregates were formed directly from hemimicelles, without the formationof bilayers. In fact, no evidence of bilayer formation was seen in this systemeven at very high surfactant concentrations.Based on the contact angle FTIR, zeta potential, and adsorption results,the preceding structural transitions are summarized by the schematic shownin Fig. 25, which illustrates the structures present at the interface in theconcentration regions AF in Fig. 24.A. Control of the Repulsion BarrierUsing CosurfactantsIn bulk micellization processes, it has been proposed that oppositely chargedsurfactant incorporates itself into micelles and by reducing the repulsionbetween the ionic groups increases stability and lowers the bulk cmc [82,83].A similar process can be expected to occur at the solidliquid interface. Asdepicted in Fig. 26, very small additions of SDS were observed to have adramatic effect on the formation of the surfactant surface structures. Figure26 shows the correlation of suspension stability of silica particles with themaximum repulsive force measured against a silica plate in the presence of3 mM C12TAB and 0.1 M NaCl at pH 4 as a function of addition of SDS.At 5 m SDS addition, no repulsive force is observed between the surfaces.However, at 10 M SDS, strong repulsive force has developed and continuesto increase with increase in SDS concentration. Correspondingly, over anidentical range of SDS concentration, the initially unstable suspension be-comesstabilized. 54. 32 Shah and MoudgilFIG. 25 Schematic representation of the proposed self-assembled surfactant filmsat concentrations corresponding to AF in Fig. 24. (a) Individual surfactant adsorp-tion,(b) low concentration of hemimicelles on the surface, (c) higher concentrationof hemimicelles on the surface, (d) hemimicelles and spherical surfactant aggregatesformed due to increased surfactant adsorption and transition of some hemimicellesto spherical aggregates, (e) randomly oriented spherical aggregates at onset of stericrepulsive forces, and (f) surface fully covered with randomly oriented sphericalaggregates. (From Ref. 43.)The abnormally low concentration of SDS needed to form the surfacesurfactant structures may be explained by the preferential adsorption of SDSinto self-assembled C12TAB aggregates at the silica surface. Adsorption ex-periments,using total organic carbon analysis to determine the total amountof adsorbed surfactant and inductively coupled plasma spectroscopy to de-terminethe SDS concentration through the sulfur emission line, have shownthat nearly all the SDS added adsorbed at the solidliquid interface. Hence,the molecular ratio at the interface was estimated to be on the order of1:10 instead of 1:100 in bulk solution. This is particularly interesting because 55. Molecular Interactions at Interfaces 33FIG. 26 Turbidity of a 0.02 vol% suspension of sol-gelderived 250-nm silicaparticles after 60 min in a solution of 0.1 M NaCl at pH 4 with 3 mM C12TAB asa function of SDS addition and the measured interaction forces between an AFMtip and silica substrate under identical solution conditions. (From Ref. 42.)it was found that no measurable quantity of SDS adsorbed onto silica in theabsence of C12TAB. In addition, because little SDS is present in solution,the system is far below the bulk cmc and yet strong repulsive forces areonce again observed.The use of cosurfactants or other coadsorbing reagents is a critical factorin the utility of a surfactant dispersant in industrial processes. Not only canthe concentrations for effective stabilization be reduced, but also many otheroptions can become available to control the overall dispersion of single- andmulticomponent suspensions. Availability of these engineered dispersant sys-temscan enhance the processing of nanoparticulate suspensions for emerg-ingspecialized end uses, such as chemicalmechanical polishing of siliconwafers in microelectronics manufacturing.VII. STABILIZATION OF CHEMICALMECHANICALPOLISHING SLURRIES UTILIZINGSURFACE-ACTIVE AGENTSChemicalmechanical polishing (CMP) is a widely used technique in mi-croelectronicdevice manufacturing to achieve multilevel metallization (Fig.27). In the CMP process, the wafer surface (on which the microelectronicdevices are built) is planarized by using a polymeric pad and a slurry com- 56. 34 Shah and MoudgilFIG. 27 (Top) Schematic representation of chemicalmechanical polishing (CMP)process. (From http://www.el.utwente.nl/tdm/mmd/projects/polish/index.html.) (Bot-tom)Review of tungsten CMP: (a) silica (interlayer dielectric) is etched, (b) tungstenis deposited onto silica ILD, and (c) CMP is applied to remove excessive tungstenlayer and other levels are built on this level (multilevel metallization). (From Ref.90.)posed of submicrometer-size particles and chemical. The ultimate goal ofCMP is to achieve an optimal material removal rate while creating an atom-icallysmooth surface finish with a minimal number of defects. This can beaccomplished by the combined effect of the chemical and mechanical com-ponentsof the process. The mechanical action in CMP is mostly providedby the submicrometer-size abrasive particles contained in the slurries as theyflow between the pad and the wafer surface under the applied pressure. Thechemical effect, on the other hand, is provided by the addition of pH reg- 57. Molecular Interactions at Interfaces 35ulators, oxidizers, or stabilizers depending on the type of the CMP operation,which makes it easy for the reacted surface to be removed by abrasive par-ticles.As the rapid advances in the microelectronics industry demand a contin-uousdecrease in the sizes of the microelectronic devices, removal of a verythin layer of material with atomically flat and clean surfaces has to beachieved during manufacturing [84]. These trends necessitate improved con-trolof the CMP process by analyzing the slurry particle size distribution andstability effects on polishing. Past investigations suggest the use of mono-sizedparticles for the CMP slurries to achieve a planarized surface and tominimize the surface deformation [85]. However, in practical applicationsthere may be oversize particles in the slurries in the form of hard-core largerparticles (hard agglomerates) or agglomerates of the primary slurry particlesbecause of slurry instability (soft agglomerates). Polishing tests conductedin the presence of hard agglomerates in the CMP slurries verified significantdegradation in the polishing performance [86]. To remove the hard agglom-erates,filtration of slurries is commonly practiced in industrial CMP oper-ations.Nevertheless, even after filtering the slurries, the defect counts onthe polished surfaces have been observed to be higher than desired [87].This observation suggested the possibility of formation of soft agglomeratesduring the polishing operations. Indeed, it was reported that the commercialCMP slurries tended to coagulate and partially disperse during polishing[88]. Figure 28 shows AFM images of silica wafers polished with soft ag-glomeratedbaseline silica slurries of 0.2 m monosize (at pH 10.5). It wasobserved that even the soft agglomerates resulted in significant surface de-formations[89], indicating that the CMP slurries must remain stable to ob-tainoptimal polishing performance.A. Stabilization of Alumina Slurries Using MixedSurfactant Systems for TungstenChemicalMechanical PolishingIn CMP processes, polishing slurries have to be stabilized in extreme en-vironmentsof pH, ionic strength, pressure, and temperature. In tungstenCMP, high concentrations of potassium ferricyanide are used to enhancesurface oxidation of tungsten. These species reduce the screening lengthbetween the alumina particles of the CMP slurries to near zero, allowing forrapid coagulation of particles and destabilization of dispersions. It has beenshown by Palla [90] that addition of a mixture of ionic (SDS) and nonionic(Tween 80) surfactants can stabilize alumina particles in the presence ofhigh concentrations of charged species. Figure 29 illustrates schematicallythe mechanism of stabilization, which can be explained as enhanced ad- 58. 36 Shah and MoudgilFIG. 28 Surface quality response of the silica wafers polished with (a) baseline0.2-m size, 12 wt% slurry; (b) dry aggregated slurry; (c) PEO flocculated slurry;(d) NaCl coagulated slurry. The inverted triangles in the AFM images (left) showthe locations corresponding to the sample roughness plots (right). (From Ref. 89.) 59. Molecular Interactions at Interfaces 37FIG. 29 Mechanisms of slurry stabilization with SDS (anionic) and Tween 80(C18PEO, nonionic) surfactants. (From Ref. 90.)sorption of nonionic surfactant using the strongly adsorbing ionic surfactantas a binding agent. The stabilizing ability of the surfactant system was foundto increase with increasing hydrophobicity of both the nonionic and ionicsurfactants. The effect of surfactant concentration on stability is shown tohave an optimal concentration range for a number of surfactants [90]. Asshown in Fig. 30a and b, when the tungsten polishing was conducted usingslurries stabilized by the described mixed surfactant system, 30% less ma-terialremoval was obtained compared with the baseline slurry; however,much better surface quality was obtained [91].B. Stabilization of Silica CMP Slurries UtilizingSelf-Assembled Surfactant Aggregates:Role of ParticleParticle and ParticleSurfaceInteractions in CMPAs discussed in the previous section, self-assembled C12TAB, a cationicsurfactant, provided stability to silica suspensions at high ionic strengths byintroducing a strong repulsive force barrier [78,79]. This novel concept has 60. 38 Shah and MoudgilFIG. 30 (a) Material removal rate response of the tungsten CMP slurries stabilizedwith mixed surfactant systems (SDS and Tween 80). (b) Surface roughness responseof the tungsten CMP slurries stabilized with mixed surfactant systems (SDS andTween 80). (From Ref. 91.) 61. Molecular Interactions at Interfaces 39been applied to stabilize the coagulated silica CMP slurries in the presenceof 0.6 M NaCl. The C12TAB surfactant was used at 1, 8, and 32 mM con-centrationsaccording to previous findings reported by Adler et al. [78] andSingh et al. [79]. Figure 31 shows the mean particle size analyses of thebaseline, 0.6 M NaCl, and 0.6 M NaClC12TAB slurries [92]. Addition of0.6 M NaCl destabilized the baseline CMP slurry by screening the chargesaround the silica particles at pH 10.5. Therefore, the mean size of the slurryincreased to 4.3 m from the baseline size of 0.2 m. Addition of 1 mMC12TAB further increased the mean particle size because the positivelycharged surfactant can screen more charges by adsorbing onto the silicaparticles. As described earlier, a jump was reported in the repulsive forcebarrier based on AFM force measurements at 8 mM C12TAB, which wasexplained on the basis of the strength of the self-assembled surfactant ag-gregatesas they formed between the AFM tip and the substrate [78,79]. Asthe repulsive force is increased, the slurry particles are expected to startstabilizing in the presence of 8 mM C12TAB. In agreement with these find-ings,the mean size of the slurry with 8mM C12TAB started to decrease,indicating that the stabilization had been initiated [92]. Finally, addition of32 mM C12TAB completely stabilized the 0.6 M NaClcontaining polishingslurry as enough repulsive force for particleparticle interaction wasreached. Figure 32a summarizes the surface quality response in terms ofFIG. 31 Mean particle size analysis of the following slurries: baseline (Geltech 0.2m, 12 wt%, pH 10.5), baseline 0.6 M NaCl, and baseline 0.6 M NaCl1, 8,or 32 mM C12TAB. The high-ionic-strength slurry is stable only at 32 mM C12TABaddition. (From Ref. 92.) 62. 40 Shah and MoudgilFIG. 32 (a) Surface quality response of C12TAB system. (b) Material removal rateresponse of C12TAB system. (From Ref. 92.)surface roughness and maximum surface deformation (the maximum depthof the scratches or pits detected on the polished wafer surface) of the waferspolished with the preceding slurries. It is clear that the surface quality im-provessignificantly for the wafers polished with the stable slurry (containing32 and mM C12TAB) as compared with the unstable slurries.After stability was achieved for the high-ionic-strength CMP slurry byadding 32 mM C12TAB, polishing experiments were conducted to measurethe material removal rate response of the surfactant-containing slurries. Fig-ure32b illustrates the material removal rates obtained in the presence ofC12TAB relative to baseline and 0.6 M N